rt2800: add chipset revision RT5390R support

About 70% of the chips with revision RT5390R initialize incorrectly, using
the auxiliary antenna instead of the main one. The net result is that
signal reception is very poor (no AP further than 1M).

This chipset differs from RT5390 and RT5390F by its support of hardware
antenna diversity. Therefore antenna selection should be done
differently, by disabling software features and previously selected
antenna.

This changeset does just that, and makes all RT5390R work properly.

This is based on Ralink's 2012_03_22_RT5572_Linux_STA_v2.6.0.0_DPO
driver.

Signed-off-by: Anisse Astier <anisse@astier.eu>
Signed-off-by: John W. Linville <linville@tuxdriver.com>